Abstract:
Ocean anoxic events are characterized by the massive burial and preservation of organic matter in association with low oxygen levels in the ocean. OAEs occurred periodically throughout Earth history. While the cause of OAEs is debatable, one scenario is increased primary productivity. The goal of this research is to better understand the relationship between nutrients and the initiation of an OAE. The primary shuttle of P to the seafloor is organic matter, linking the burial of C and P. P is also the limiting nutrient on geologic time scales; therefore, we are studying the detailed P geochemistry at one site in the south Atlantic Ocean, (Demerara Rise). Samples from Site 1258 were collected as part of Ocean Drilling Program Leg 207. Samples were analyzed at/near the occurrence of OAE II (Cenomanian-Turonian boundary approximately 93 Mya). A sequential extraction was performed at 204 different sample intervals. Results show that the average P components are as follows: 0% oxide-associated, 82% authigenic/biogenic, 27% detrital, and 4% organic. The low abundance of oxide-associated P suggests it was likely lost to the overlying water column, increasing the P concentration of the bottom waters. Upwelling of this P-rich deep water elsewhere could generate more primary productivity, which likely led to further anoxia. In addition, the low concentrations of organic P indicates sediments have undergone significant diagenetic redistribution.